1. Field of the Invention.
The present invention relates generally to switching circuits for switching terminals between high and low impedance states. In particular, the present invention is a bipolar AC switch for controlling frequency characteristics of a loop filter in a phase-locked loop.
2. Description of the Prior Art.
Phase-locked loops or PLL's are commonly used circuit elements. A typical phase-locked loop includes a phase detector which compares the frequency of an output signal generated by a voltage controlled oscillator with that of an input signal. The output of the phase detector is passed through a loop filter and fed to the voltage controlled oscillator. The voltage controlled oscillator thereby produces an output signal with a relative phase identical to that of the input signal. Frequency characteristics of the phase-locked loop are determined by the loop filter.
It is often desirable to selectively control the frequency characteristics of the loop filter. For example, to produce a phase-locked loop capable of quickly locking to the phase of input signals it is desirable to have a loop filter with high gain. However, when the loop filter has high gain, the output signal of the PLL is very sensitive to phase changes in the input signal and relatively unstable. Conversely, although a phase-locked loop with a low bandwidth filter will have a more stable output, its lock acquisition time is greater.
Phase-locked loops having switchable frequency characteristics are well known in the art. High bandwidth for fast acquisition is desired when starting the loop, while low bandwidth and stability are preferred during normal operation. Typically, the phase comparator and voltage controlled oscillator are fabricated on an integrated circuit, while the loop filter is formed by various resistors and capacitors external to the integrated circuit. A FET or other switching device, also external to the integrated circuit chip, is controlled by logic signals and switches a circuit element such as a resistor into or out of an RC network to change the bandwidth of the loop filter. In arrangements of this type, the FET is often connected between the switched circuit element and ground. When switched OFF it must provide a high impedance, while functioning as both an AC and DC ground when switched ON. As such, it is important that its offset voltage to ground be low so that the filter characteristics can be predicted.
The use of external circuit elements such as the switching FET described above is always undesirable since they require additional space and add additional expense beyond that of the integrated circuit. Although phase-locked loops which are fabricated on integrated circuits using CMOS or other technologies which will support a FET can have the FET fabricated thereon, this option is not available for PLLs fabricated using bipolar technology. Since bipolar transistors will only support DC current, more complicated switching arrangements must be utilized.
It is evident that there is a continuing need for digitally-controlled AC switching networks, especially those suitable for fabrication with bipolar technology. The AC switch should have a fast response time, and provide high output impedance in its OFF state with low leakage current. Of course, it must also be fabricated using as few devices as possible so as to be relatively inexpensive and require little area of the integrated circuit.